Cathode ray oscillograph tube



Nova 29, 1938. H. BEDFORD CATHODE RAY OSCILLOGRAPH TUBE 2 Sheets-Shee't 1 Filed Jan. 9, 1936 A T'I'OR NEYS Nov, 29, 1938. 1-1. 'BEDFORD 2,138,721

CATHODE RAY OSCILLOGRAPH TUBE 2 Sheets-Sheet 2 F iled Jan. 9, 1936 INVENTOK LESLIE H. BEDFORD B y fllH-Pwv U III ATTORNEYS Patented Nov. 29, 1938 GATHODE RAY OSCILLOGRAPH TUBE Leslie Herbert Bedford,

England, assignor to A.

don, England Highbury Grove, London,

0. Cossor Limited, Lon- Application January 9, 1936, Serial No. 58,408

In Great Britain 3 Claims.

This invention relates to cathode ray tubes and more particularly to those of the so called hard vacuum variety in which a system of electrodes having electron-optical lens properties is employed for focusing the electron beam so as to produce a spot on the screen.

The principal object of the present invention is to improve the electron optical lens system in such a way as to permit of the concentration of a comparatively large ray current upon a small spot whose sharpness and size are not seriously affected by the functioning of the conventional ray modulating means provided for modulating the ray current.

According to the invention, with this object in view, the electron optical system is arranged so that the spot produced on the screen is an image of the cathode and is substantially unaffected as to size and sharpness by modulation of the electron beam.

The electron optical system includes a first electron lens which concentrates the electron emission into the form of a beam in such a way as to produce a virtual image of the cathode at a point behind the cathode. A further electron lens system is arranged to focus the aforesaid virtual image on the screen. A modulating electrode adapted to modulate the electron beam by its influence on the space charge near the oathode is provided, and this modulating electrode effectively forms part of the first electron lens system since it necessarily influences the electrostatic field thereof. The functioning of this modulating electrode inevitably causes variations in L the focal length of the first electron lens system Which result in variations in the size and position of the virtual image produced. According to an important feature of the present invention, in order to minimize these unavoidable variations, the first electron lens system is placed close to the cathode, so that the magnification produced by the first electron lens system is reduced to a minimum. This results in strictly limiting the variations in size and position of the virtual image of the cathode due to modulation, so that in effect a sharply defined spot of nearly constant size is maintained throughout the whole range of modulation.

In the preferred arrangement, the complete electron optical system comprises a modulating electrode in the form of a Wehnelt cylinder and three anodes of which the first and third take the form of a perforated disc and are spaced apart by the second anode which is of tubular formation. The modulating electrode and first anode January 24, 1935 constitute the first electron lens system above referred to and, in accordance with the principle which has just been explained, the cathode is situated at a short distance behind this anode n so that the magnification due to the first electron lens system is small. In order to give the modulating cylinder a good modul the ray current in spite of th between the cathode and the ation control of e short distance first anode it is necessary to make the modulating cylinder of small diameter.

For example, with a distance of 1.5 mm. between the cathode and the first anode,

the diameter of the modulating of the order of 5 mm.

cylinder may be The cathode preferably comprises a member which is heated from a filamentary element and which is so shaped and coated w ith emitting material as to produce a point-like source of electron current.

This member which acts as the cathode may comprise a short cylindrical met allic rod of small diameter which is placed in metallic contact with the filamentary element so as to be heated from the latter by thermal conduction, the longitudinal axis of the rod coinciding with the longitudinal axis of the tube, and the end surface of the rod facing the elctrode system being coated with an electron emitting material.

invention.

Fig. 2 is a side View of an electrode assembly such as might form part of a cathode ray tube constructed in accordance with the invention. Fig. 3 is an enlarged side view of the cathode construction and Fig. 4; is a diagrammatic representation of an optical arrangement analogous to an electron lens system such as that constituted by the electrode arrangement shown in Figure l or 2. Like parts are indicated by like reference numbers throughout.

Referring first to Figure 2, the electrode assembly including a tube having a fluorescent screen H (see Fig. 1) is supported by the four wings l2 of a glass pinch l3, these wings being arranged in substantially cruciform manner as described in United States Letters Patent No.

issued Sept. 11, 1934.

The cathode,

which is clearly shown in Figure 3, consists of a short cylindrical platinum rod l gitudinal axis of the tube by m mentary heating wire 2 welde held in the loneans of the filad to supporting having a flanged periphery and a small aperture at the center immediately above the cathode I. The anode 4 is insulated from the modulator cylinder 3 by means of mica discs bolted to the underside of the anode 4, one of these discs fitting round the'upper end of the modulating cylinder so as to hold the latter firmly in position.

The first anode 4 is held in position by means of four rods l6, l1, l8 and I9 which are sealed into the four arms of the pinch and pass through apertures in the anode 4 where they make close contact with the mica disc previously described. These four rods I 6, I1, 18 and I9 pass upwards beyond the first anode 4 and support a further 'ported by a short metallic rod.

anode 6 this anode being held by the four rods in the same way as the anode 4. The anode 6 is also provided with an aperture in the center and this may be termed the third anode since it is separated from the first anode 4 by a further anode in the form of a cylindrical metallic tube 5 which may be termed the second anode. This second anode 5 is insulated from the first anode 4 by a mica ring I 5 bolted to the upper side of the anode '4 and from the third anode 6 by another mica ring arranged within a downwardly extending tubular portion or skirt [6 formed on the lower side of the third anode 6. The mica supporting ring is preferably arranged a short distance down the skirt It; for example, with a skirt 23 min. in diameter and a tubular anode 13 mm. in diam- .eter, the supporting mica ring may be placed at a distance of 5 mm. from the top of the skirt.

The third anode 6 is surmounted by two pairs of deflector plates 1, 8 and 9 and II] for deflecting the ray which emerged from the third anode 6. One side of each of these four deflector plates is supported by one of the rods l6, 11, I8 and I9, while the other side of each of the plates is sup- These short metallic rods, one of which is shown at 29, pass through the third anode 6 and are secured to an.

annular mica ring 2| fixed to the rods l6, l1, l8 and I9. Operating voltage is supplied to the first anode 4 via a lead 22, to the second anode 5 via a lead 23 and to the third anode 6 Via a lead 24 which is attached to the skirt portion [6 of the anode. The leads 23 and 24 are surrounded through the greater part of their length by small diametered glass tubes.

In the case of small or medium sized cathode ray tubes each of the deflector plates is provided with a strip of magnesium ribbon 25 which is held in position by gauze 26. When this magnesium is fired during the gettering operation a metallic deposit is formed on the inner walls of the tube surrounding the deflector plates. During the subsequent operation of the tube this metallic deposit may be held at the same potential as the third anode 6 by means of an extending spring member 21 attached to the anode 6 and arranged to make contact with the metallic deposit. In the case of large sized tubes requiring a greater amount of metallic deposit, however, the two supporting members 28 (shown in broken anode 6 a voltage of 300. The modulating electrode may be biassed to a Voltage of 200.

The theory of the system can perhaps be best understood by a reference to Figure 4 in which an optical arrangement is shown, analogous to the skeleton electrode system shown in Figure 1.

In Figure 4 the arrow C represents the cathode, and F1 and F2 represent two converging lenses.

A virtual image V1 of the cathode C is produced by the first lens F1 and a real image I of this virtual image is produced by the second lens F2, this real image being arranged 'to coincide with the position of the screen. The arrangement of the lens together with the position of the cathode C is such that the virtual image V1 is formed with a minimum of magnification, the cathode C being placed as close to the lens F1 as possible.

Returning now to the cathode ray tube arrangement, the electric field between. the modulating cylinder 3 and the first anode 4 may be regarded as constituting the first lens (equivalent to the lens F2 of Figure 4) which results in a virtual image of the cathode I being formed at some distance behind it. The field between the second anode 5 and the third anode 6 constitutes the second electron lens by means of which a real image of the virtual image above mentioned is formed on the fluorescent screen. As previously explained, the distance between the cathode l and the anode is made as small as possible. This results in strictly limiting the variations in size and. position of the virtual image of the cathode 1 due to modulation n the cylinder so that in effect a sharply defined spot of nearly constant size is maintained throughout the whole range of modulation.

In order to give the cylinder 3 a good modulation control of the ray current in spite of the short distance between the cathode l and the anode 4 it is necessary to make the cylinder 3 of small diameter. For example, with a distance of 1.5 mm. between the cathode l and the anode 4, the diameter of the cylinder 3 may be of the order of 5 mm.

I claim:

1. An electron-optically focussed cathode ray tube of the type wherein the cathode is located adjacent the rear end and a screen is disposed adjacent the front end comprising in combination an actual cathode having an emitting surface which is substantially independent in magnitude of ray current, a first accelerating electrode, a further electrode located behind said first accelerating electrode and creating in conjunction therewith a diverging electron lens immediately in front of the cathode, said first accelerating electrode being located so close to said cathode that the focal length of said lens is greater than the distance between said lens and said cathode, whereby said lens produces behind said cathode a virtual image of the whole emitting surface thereof only slightly magnified, and means to produce from said virtual image a real image of the whole emitting surface of the cathode.

2. An electron-optically focussed cathode ray tube of the type wherein the cathode is located adjacent the rear end and a screen is disposed adjacent the front end comprising in combination an actual cathode having an emitting surface which is substantially independent in magnitude of ray current strength, a first accelerating electrode and a further electrode, at least one of which electrodes constitutes a ray modulating element in close proximity to said emitting surface, said further electrode being located behind said first accelerating electrode and creating in conjunction therewith a diverging electron lens immediately in front of the cathode, said first accelerating electrode being located so close to said cathode that said lens produces behind said cathode a virtual image of the whole emitting surface thereof only slightly magnified, and means to produce from said virtual image a real image of the Whole emitting surface of the cathode.

3. In the tube structure defined in claim 2, said modulating element being formed by said further electrode and comprising a cylinder very closely 10 surrounding said cathode.

LESLIE HERBERT BEDFORD. 

